Is Partial Fractions Necessary?

Algebra Level 3

n = 1 2 n + 1 n 2 ( n + 1 ) 2 = 3 4 + 5 36 + 7 144 + 9 400 = ? \large \sum_{n=1}^\infty\dfrac{2n+1}{n^2(n+1)^2} = \dfrac34 + \dfrac5{36} + \dfrac7{144} + \dfrac9{400} \cdots = \, ?


The answer is 1.

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1 solution

Rishabh Jain
Mar 26, 2016

S = n = 1 ( 2 n + 1 n 2 ( n + 1 ) 2 ) \large\mathfrak{S}= \sum_{n=1}^\infty\left(\dfrac{2n+1}{n^2(n+1)^2}\right)

S = n = 1 ( ( n + 1 ) 2 n 2 n 2 ( n + 1 ) 2 ) = n = 1 ( 1 n 2 1 ( n + 1 ) 2 ) \large {\begin{aligned}\mathfrak S&=\sum_{n=1}^\infty\left(\dfrac{(n+1)^2-n^2}{n^2(n+1)^2}\right) \\&=\sum_{n=1}^\infty\left(\dfrac{1}{n^2}- \dfrac{1}{(n+1)^2}\right) \end{aligned}}

( A T e l e s c o p i c S e r i e s ) \large(\mathbf{A~Telescopic~Series})

= 1 1 2 1 2 2 + 1 2 2 1 3 2 + 1 3 2 =\dfrac{1}{1^2}-\cancel{\dfrac{1}{2^2}}+\cancel{\dfrac{1}{2^2}}-\cancel{\dfrac{1}{3^2}}+\cancel{\dfrac{1}{3^2}}\cdots

= 1 1 2 = 1 \huge =\dfrac{1}{1^2}=\boxed 1

Hey , what is that greek letter called? By the way nice solution

Aakash Khandelwal - 5 years, 2 months ago

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No thats not a greek letter :-)......... Use \mathfrak{S} for that....

Rishabh Jain - 5 years, 2 months ago

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